The present invention relates to image processing, and more particularly, to a method and apparatus for canceling chrominance aberration.
Chromatic Aberration (CA) is an aberration caused by a difference in refractive index according to different wavelengths of light. In an ideal lens, white light that is incident on the lens should pass through the lens and then be focused on one point on an image plane. However, in an actual lens, light having relatively short wavelength is refracted more than light having relatively long wavelength and thus a focus varies according to the wavelengths of lights due to the difference in refractive index according to the different wavelengths. That is, a focus varies according to colors of light. A color image is formed of three colors, i.e., red (R), green (G), and blue (B). As described above, in general, light of R.G.B focuses on different spots due to an effect of the lens and thus, false colors are generated in the image. This is referred to as “chromatic aberration”.
Chromatic aberration may be mainly classified into longitudinal chromatic aberration and lateral chromatic aberration. Longitudinal chromatic aberration is caused due to a focus difference on an optical axis, and lateral chromatic aberration is caused due to a difference in magnification according to color. Longitudinal chromatic aberration and lateral chromatic aberration are not generated separately and instead, are generated together.
Chromatic aberration may be observed in most optical devices using a lens. In manufacturing of the lens used in the optical devices, various lenses are combined to correct the chromatic aberration. However, even if the lenses are combined, chromatic aberration cannot be completely cancelled. Also, in most cameras installed in mobile phones and typical compact cameras, inexpensive lenses are used and thus, chromatic aberration may be more conspicuous. Moreover, although resolutions of cameras installed in mobile phones and digital cameras are rapidly increasing, lens quality does not proportionally increase due to cost and size of the lenses. That is, chromatic aberration is generated mostly in cameras, and manufacturers of the cameras are trying to reduce chromatic aberration.
A conventional method of canceling chromatic aberration includes matching sizes of three channels, R, G, and B, in order to correct chromatic aberration occurring due to lateral chromatic aberration. In order to match the sizes of the three channels, an interpolation process is required, in which displacement in each channel is predicted, a proportion of a size of an image is obtained through predicted information, and the sizes of two channels are matched to the size of the remaining single channel. Here, scaling parameters of the R, G, and B channels are calculated according to a registration algorithm, the registration algorithm using a black and white image having a specific pattern. That is, corner points having the specific pattern are extracted and matched for each channel and then, scaling information is identified using the corner points.
In the conventional method, lateral chromatic aberration can be efficiently cancelled as the sizes of the images are matched by predicting displacement of the channels. However, if the conventional method is used, registration information must be previously predicted for each lens through the specific pattern and then, the chromatic aberration is cancelled using the parameters (that is, registration information must be known for all lenses in advance), and extracting the parameters and canceling chromatic aberration are separated since the registration algorithm must be performed independently by using the image having the specific pattern. In addition, the conventional method is not effective in DSLR cameras in which lenses are changed. Furthermore, DSLR cameras have limitations in correcting chromatic aberration caused by longitudinal chromatic aberration and lateral chromatic aberration together since longitudinal chromatic aberration is not considered in DSLR cameras.
An analysis of an image that undergoes chromatic aberration reveals that there is directionality in chromatic aberration occurring according to the location of the image, particularly, due to lateral chromatic aberration. That is, the directionality of chromatic aberration in a current pixel that is to be processed may be determined on a block-by-block basis of the image. Thus, it is possible to effectively cancel chromatic aberration by performing filtering a region of the image, which is asymmetrical with respect to a direction in which chromatic aberration occurs.
Also, when chromatic aberration occurs in a location in an image, on which strong light is incident, the size of the location increases. However, in order to perform a technique of canceling such chromatic aberration in a hardware manner, there is a limit to the number of line memories that can be used, and particularly, to canceling chromatic aberration occurring in the vertical direction. Thus, when the total number of line memories that can be used is insufficient to cancel chromatic aberration occurring in the vertical direction, chromatic aberration is difficult to cancel. Accordingly, in order to solve this problem, there is a need for development of a method of efficiently canceling chrominance aberration by using a given number of line memories.
Also, in general, an image is focused around the focus of yellow light. That is, the location of an image plane focused when light is incident on and passes through the lens is present between the focuses of signals from G and R channels. In this case, since a signal from a B channel is focused far from the image plane, an image from the B channel is more blurred than those from the R and G channels, and thus, blue chromatic aberration is more visible in the image from the B channel than in images from the R and G channels. Accordingly, there is a need to effectively cancel chromatic aberration in the B channel.